It is known to provide multifunction liquid crystal screens in the visual display systems of aircraft cockpits, where such screens are capable of displaying a large amount of information simultaneously. The information may have a variety of presentations:                text and simple crosshairs generated by the display unit;        a bitmap type colored background provided by a system external to the visual display system; and        a complex video image also provided by an external system.        
In order to optimize graphics processing and to guarantee correct priority levels for the symbology generated by the visual display system, it is known that the graphics memory that defines the content of the digital image for display on the matrix of the screen may be organized as a plurality of graphics planes, specifically:                a symbology plane;        a matrix or bitmap plane; and        a video plane.        
Under such circumstances, it is possible to display on the screen an image that presents a background of strong graphic density having signs associated with some particular symbology superposed thereon. By way of example, the background may be a map, a radar image, information provided by a database relating to terrain, or indeed a satellite image. The function of the background is to improve the aircraft pilot's perception of the surroundings outside the aircraft. Nevertheless, it does not constitute primary means for guaranteeing flight integrity.
The symbology may be associated with information relating to a variety of domains such as the flight plan, radio-navigation, managing on-board systems, sending messages or alarms, or indeed elements concerning monitoring the surrounding air space. Such elements may for example relate to traffic alert and collision avoidance systems (TCAS).
This information is generally more critical than the background image, and in any event of higher priority for flight integrity. That is why the way in which the graphics planes are organized ensures that the symbology is always drawn so as to be visible in front of the background image, i.e. on top of it.
Nevertheless, that does not suffice to ensure that the symbology is clearly legible under all circumstances, in particular if the background is moving or cluttered with numerous graphics elements of different colors.
Unlike raster-scanning cathode ray screens where superposing signs has the result of increasing the light intensity of each pixel, thereby ensuring satisfactory contrast in all circumstances, with matrix type screens such as liquid crystal screens, such superposition involves a priority order. Thus, at each pixel of the image, only the element corresponding to the graphics plane having the highest priority is displayed. In other words, only the color (and consequently the luminance) of the sign having the highest priority is displayed amongst signs that are superposed.
Under such conditions, since liquid crystal screens are nowadays greatly preferred over raster-scanning cathode ray screens, it is important to make priority information clearly visible on the image.
One known solution for improving the legibility of the symbology on the background is haloing. That consists in emphasizing the outline of the priority item (e.g. a character or crosshairs) by a black line that artificially increases the contrast between the item and the background that surrounds it. Nevertheless, that technique is effective above all when the signs are on a colored background that is relatively uniform or when the sign forms solid crosshairs. It is found to be much less effective with characters, or thin traces that are displayed on a background presenting high resolution.